1. Field of the Invention
The present invention relates to logic simulation for a printed circuit board unit mounted with such a device as a microprocessor whose internal logic is not known, an LSI or others.
2. Description of the Related Arts
A method of obtaining test data of a device mounted on a printed circuit board unit whose internal logic is not known, by using the device itself, is described, for example, in JP-A-56-2046. According to this method, an emulator using the actual device itself is provided as a software fault simulator to operate the emulator with its input and output correlated with each other so that test data can be obtained. However, in logic simulation, if signals applied to the input pins of an LSI whose internal logic is not known change during simulation procedure, the corresponding output signals are required every time such a change occurs. Thus, this method is not available for logic simulation.
The detail of the internal logic of an LSI is not known in most case except for custom LSI's designed at its own factory. It is difficult to deduce the internal structure of a large scale microprocessor based on its function, and even if it is possible it remains no better than deduction.
As a logic simulation method for a logic circuit including an LSI whose internal logic is not known, there is a method of describing the function of an LSI by using software. According to this method, since the function and operation of an LSI is known although the internal logic is not available, the function and operation is logically expressed and defined to operate the LSI using the software and obtain an expected output data for a certain input data. With this method, it is necessary to use a different definition of function and operation for each type of LSI's whose internal logic are not known. Particularly, in case of an LSI which is designed to perform complicated function, such definition work as well as development effort for behavioral description language requires highly sophisticated knowledge and skill, and takes a long time, resulting in a very difficult work. Further, it is necessary to confirm that the output value from the software model is consistent with that of the actual LSI. Therefore, debug work for this software model requires an extremely long time.
In an alternative logic simulation method, an LSI itself whose internal logic is not known is positively used. The objective LSI is connected to a computer system performing software logic simulation via an I/O device. When the signal values to the LSI change during a logic simulation procedure, the LSI is called in a similar manner to the case of a call subroutine to transfer the corresponding output signals to the software logic simulator. Thus, logic simulation is performed for a logic circuit including logical elements whose internal logic are not known.
In such a logic simulation using an actual device, it is necessary to consider timings between hardware and software. Particularly in the case of an LSI using dynamic MOS circuits, the internal circuits thereof are not operable without a minimum operating speed. Therefore, data transfer from the software to the actual device may become insufficient without considering this minimum operating speed. To solve this problem, there is known a method wherein all the input signal values to the LSI are saved as a history from the start of logic simulation, and every time the actual device is called in accordance with software logic simulation, the history and a new input value are sequentially executed at a higher speed than the minimum operation speed to obtain the corresponding outputs. With this method, although the minimum operating speed is ensured, since the history from the start of logic simulation must be executed every time the actual device is called, a long time in simulation and hence in execution of the history is required and the memory capacity must be increased.